1. Field of the Invention
The present invention relates to a method of manufacturing a semiconductor film having a crystal structure, formed on a substrate having an insulating surface, and a method of manufacturing a semiconductor device using the semiconductor film as an active layer. In particular, the present invention relates to a method of manufacturing a thin film transistor having an active layer formed of a crystalline semiconductor. In this specification, a semiconductor device generally refers to those capable of functioning by utilizing semiconductor characteristics, and includes an electro-optical device such as an active matrix type liquid crystal display device formed by using a thin film transistor, and electronic equipment provided with such an electro-optical device as a component.
2. Description of the Related Art
A technique has been developed, in which an amorphous semiconductor film is formed on a light-transparent substrate having an insulating surface, and a crystalline semiconductor film obtained by crystallizing the amorphous semiconductor film by laser annealing, thermal annealing, or the like is used as an active layer for a thin film transistor (hereinafter, referred to as a xe2x80x9cTFTxe2x80x9d). As the light-transparent substrate having an insulating surface, in most cases, a glass substrate made of barium borosilicate glass, aluminoborosilicate glass, or the like is used. Although such a glass substrate has poor heat resistance, compared with a quartz substrate, it is inexpensive. Furthermore, a glass substrate has the advantage of enabling a substrate with a large area to be easily produced.
Laser annealing is known as a crystallization technique that gives high energy only to an amorphous semiconductor film to crystallize it without substantially increasing the temperature of a glass substrate. In particular, an excimer laser that emits light with a short wavelength (400 nm or less) is a representative laser that has been used from the beginning of the development of laser annealing. In recent years, a technique using a YAG laser that is a solid-state laser has also been developed. According to laser annealing using these lasers, a laser beam is formed by an optical system so as to have a spot shape or a linear shape on a surface to be irradiated, and the surface to be irradiated on the substrate is scanned by the resultant laser light (i.e., an irradiation position of laser light is moved relative to the surface to be irradiated). For example, according to excimer laser annealing using linear laser light, the entire surface to be irradiated can be subjected to laser annealing by scanning only in a direction orthogonal to a longitudinal direction, and such laser annealing is excellent in productivity. Therefore, excimer laser annealing is becoming the mainstream in production of a liquid crystal display device using TFTs. This technique realizes a monolithic liquid crystal display device in which TFTs forming a pixel portion (pixel TFTs) and TFTs for a driver circuit provided on the periphery the pixel portion are formed on one glass substrate.
However, a crystalline semiconductor film formed by subjecting an amorphous semiconductor film to laser annealing includes a collection of a plurality of crystal grains, and the position and size of the crystal grains are random. TFTs are formed on a glass substrate by patterning a crystalline semiconductor layer in an island shape for device separation. In this case, the position and size of crystal grains cannot be specified. It is known that an interface of crystal grains (grain boundary) involves factors that cause current transporting characteristics of carriers to be degraded, due to the influence of a recombination center or a trapping center caused by an amorphous structure, a crystal defect, and the like, and the influence of a potential level at a grain boundary. However, it is almost impossible to form a channel formation region, crystal properties of which have a serious effect on the TFT characteristics, using single crystal grains while avoiding the influence of a crystal boundary. Therefore, a TFT using a crystalline silicon film as an active layer has not been obtained, which has characteristics equivalent to those of a MOS transistor formed on a single crystal silicon substrate.
In order to solve such problems, an attempt to grow a large crystal grain has been made. For example, in ┌xe2x80x9cHigh-Mobility Poly-Si Thin-Film Transistors Fabricated by a Novel Excimer Laser Crystallization Methodxe2x80x9d, K. Shimizu, O. Sugiura, and M. Matumura, IEEE Transactions on Electron Devices vol. 40, No. 1, pp 112-117, 1993┘, there is a report on a laser annealing method in which a film of three-layer structure of Si/SiO2/Si is formed on a substrate, and an excimer laser beam is irradiated from both sides of a film side and a substrate side. This report discloses that according to this method, the size of a crystal grain can be enlarged by irradiation of a laser beam at predetermined energy intensity.
The above-mentioned method of Shimizu et al. is characterized in that heat characteristics of an under material of an amorphous silicon film are locally changed and the flow of heat to the substrate is controlled, so that a temperature gradient is caused. However, for that purpose, the three-layer structure of high melting point metal layer/silicon oxide layer/semiconductor film is formed on the glass substrate. Although it is possible to form a top gate type TFT by using the semiconductor film as an active layer in view of structure, since a parasitic capacitance is generated by the silicon oxide film provided between the semiconductor film and the high melting point metal layer, power consumption is increased and it becomes difficult to realize high speed operation of the TFT.
On the other hand, when the high melting point metal layer is made a gate electrode, it is conceivable that the method can be effectively applied to a bottom gate type or reverse stagger type TFT. However, in the foregoing three-layer structure, even if the thickness of the semiconductor film is omitted, with respect to the thickness of the high melting point metal layer and the silicon oxide layer, since the thickness suitable for a crystallizing step is not necessarily coincident with the thickness suitable for the characteristics as a TFT element, it is impossible to simultaneously satisfy both the optimum design in the crystallizing step and the optimum design in the element structure.
Besides, when the opaque high melting point metal layer is formed on the entire surface of the glass substrate, it is impossible to fabricate a transmission type liquid crystal display device. Although the high melting point metal layer is useful in that its thermal conductivity is high, since a chromium (Cr) film or titanium (Ti) film used as the high melting point metal material layer has high internal stress, there is a high possibility that a problem as to adhesiveness to the glass substrate occurs. Further, the influence of the internal stress is also exerted on the semiconductor film formed as the upper layer, and there is a high possibility that the stress functions as force to impart distortion to the formed crystalline semiconductor film.
On the other hand, in order to control a threshold voltage (hereinafter referred to as Vth) as an important characteristic parameter of a TFT within a predetermined range, in addition to valence electron control of the channel formation region, it is necessary to reduce the charged defect density of an under film and a gate insulating film formed of an insulating film to be in close contact with the active layer, or to consider the balance of the internal stress. To such requests, a material containing silicon as its constituent element, such as a silicon oxide film or a silicon nitride oxide film, has been suitable. Thus, there is a fear that the balance is lost by providing the high melting point metal layer to cause the temperature gradient.
The present invention has been made to solve such problems, and an object of the invention is to realize a TFT capable of operating at high speed by fabricating a crystalline semiconductor film in which the positions and sizes of crystal grains are controlled and further by using the crystalline semiconductor film for a channel formation region of the TFT. Further, another object of the invention is to provide a technique enabling such a TFT to be applied to various semiconductor devices such as a transmission type liquid crystal display device or a display device using electroluminecence material.
Laser annealing is used for forming a crystalline semiconductor layer from an amorphous semiconductor layer formed on a substrate made of glass or the like. According to laser annealing of the present invention, a pulse oscillation type or continuous light-emitting type excimer laser, YAG laser, or argon laser is used as a light source, and laser light formed into a line shape or a rectangular shape by an optical system is irradiated to an island-like semiconductor layer through both front and back surfaces of a substrate with the island-like semiconductor layer formed thereon. In this specification, the front surface of a substrate is defined as the one on which an island-like semiconductor layer is formed, and the back surface of the substrate is defined as the one opposite to the side on which the island-like semiconductor layer is formed.
FIG. 2A shows a structure of a laser annealing apparatus according to the present invention. The laser annealing apparatus includes a laser oscillator 1201, an optical system 1100, and a stage 1202 for fixing a substrate. The stage 1202 is provided with a heater 1203 and a heater controller 1204 and is capable of heating the substrate fixed thereto up to 100xc2x0 C. to 450xc2x0 C. A reflective plate 1205 is provided on the stage 1202, and a substrate 1206 is placed on the reflective plate 1205. A method of holding the substrate 1206 in the structure of the laser annealing apparatus shown in FIG. 2A will be described with reference to FIG. 2B. The substrate 1206 held by the stage 1202 is placed in a reaction chamber 1213, and is irradiated with laser light. The reaction chamber 1213 can be put in a state of a reduced pressure or in an atmosphere of inert gas by an exhaust system or a gas system (not shown), and can heat a semiconductor film to 100xc2x0 C. to 450xc2x0 C. without contaminating it. The stage 1202 can move in the reaction chamber 1213 along a guide rail 1216, so as to allow the entire surface of the substrate 1206 to be irradiated with linear laser light. Laser light is incident through a quartz window (not shown) provided above the substrate 1206. Furthermore, in FIG. 2B, a transfer chamber 1210, an intermediate chamber 1211, and a load/unload chamber 1212 are connected to the reaction chamber 1213. The intermediate chamber 1211 is separated from the load/unload chamber 1212 by a gate valve 1217, and the reaction chamber 1213 is separated from the transfer chamber 1210 by a gate valve 1218. A cassette 1214 capable of holding a plurality of substrates is placed in the load/unload chamber 1212, and a substrate is transported by a transportation mechanism 1215 provided in the transfer chamber 1210. A substrate 1206xe2x80x2 represents a substrate which is being transported. Because of the above-mentioned structure, laser annealing is performed continuously under a reduced pressure or in an atmosphere of inert gas.
FIGS. 3A ad 3B illustrate a basic structure of the optical system 1100 in the laser annealing apparatus shown in FIG. 2A. An excimer laser, a YAG laser, an argon laser, or the like is used for a laser oscillator 1101. FIG. 3A is a side view of the optical system 1100. Laser light emitted from the laser oscillator 1101 is split in a vertical direction by a cylindrical lens array 1102. The split laser light is once condensed by a cylindrical lens 1104 and spreads. The laser light is then reflected by a mirror 1107, and formed into linear laser light on an irradiation surface 1109 by a cylindrical lens 1108. Because of this, the energy distribution of linear laser light in a width direction can be made uniform. FIG. 3B is a top view of the optical system 1100. Laser light emitted from the laser oscillator 1101 is split in a lateral direction by the cylindrical lens array 1103. Thereafter, the laser light is combined on the irradiation surface 1109 by the cylindrical lens 1105. Because of this, the energy distribution in a longitudinal direction of the linear laser light can be made uniform.
FIG. 1 illustrates an idea of laser annealing of the present invention. A stripe-like first insulating layer 1002 is formed on a substrate 1001 made of glass or the like. A second insulating layer 1003 is formed on the first insulating layer 1002. Furthermore, an island-like semiconductor layer 1004 is formed on the second insulating layer 1003. As the first and second insulating layers 1002 and 1003, a silicon oxide film, a silicon nitride film, a silicon oxide nitride film, an insulating film mainly containing aluminum, or the like can be used alone or in an appropriate combination.
Laser light that passes through a cylindrical lens 1006 having the same function as that of the cylindrical lens 1108 is irradiated to the island-like semiconductor layer 1004 as linear laser light by the optical system 1100 illustrated in FIGS. 3A and 3B. The laser light irradiated to the island-like semiconductor layer 1004 contains a first laser light component 1007 and a second laser light component 1008. The first laser light component 1007 passes through the cylindrical lens 1006 to be directly irradiated to the island-like semiconductor layer 1004. The second laser light component 1008 passes through the first insulating layer 1002, the second insulating layer 1003 and the substrate 1001, it is reflected by a reflective plate 1005, passes again through the substrate 1001, the first insulating layer 1002, and the second insulating layer 1003, and is irradiated to the island-like semiconductor layer 1004. In any case, the laser light passing through the cylindrical lens 1006 has an incident angle of 45xc2x0 to 90xc2x0 with respect to the substrate surface during condensing, so that the laser light reflected by the reflective plate 1005 is also reflected in a direction that leads to the inside of the island-like semiconductor layer 1004. The reflective plate 1005 has a reflective surface made of aluminum (Al), titanium (Ti), titanium nitride (TiN), chromium (Cr), tungsten (W), tungsten nitride (WN), or the like. In this way, by appropriately selecting a material for the reflective surface, its reflectance can be varied in a range of 20 to 90%, and the intensity of laser light incident trough the back surface of the substrate 1001 can be varied. Furthermore, when the reflective surface is formed into a mirror surface, a regular reflectance of about 90% is obtained in a wavelength range of 240 to 320 nm. Furthermore, when the reflective plate is made of aluminum, and minute unevenness of hundreds of nm is formed on its surface, a diffuse reflectance (integrated reflectancexe2x80x94regular reflectance) of 50 to 70% is obtained.
Thus, laser light is irradiated through both the front and back surfaces of the substrate 1001, and the island-like semiconductor layer 1004 formed on the substrate 1001 is subjected to laser annealing through both surfaces thereof. According to laser annealing, by setting optimum conditions of laser light to be irradiated, a semiconductor layer is instantaneously heated to be melted, to thereby control the generation density of crystal nuclei and crystal growth from the crystal nuclei. The oscillation pulse width of an excimer laser and a YAG laser which emit pulse light is several nsec to tens of nsec (e.g., 30 nsec); therefore, when a semiconductor layer is irradiated with laser light with a pulse oscillation frequency of 30 Hz, it is instantaneously heated, and cooled for a much longer period of time, compared with the heating time.
If only one surface of an island-like semiconductor layer formed on a substrate is irradiated with laser light, a cycle of melting by heating and solidifying by cooling is abrupt. Therefore, even if the generation density of crystal nuclei is controlled, sufficient crystal growth cannot be expected. However, when both surfaces of an island-like semiconductor layer are irradiated with laser light, a cycle of melting by heating and solidifying by cooling is gentle, and a time allowed for crystal growth during solidifying by cooling is relatively long; therefore, sufficient crystal growth can be obtained.
In the above-mentioned transient phenomenon, by providing an island-like semiconductor layer with a temperature distribution to obtain a region where a change in temperature is gentle, so as to control a nucleus generation speed and a nucleus generation density, crystal grains can be made larger. More specifically, as shown in FIG. 1, the stripe-like first insulating layer 1002 is provided on the substrate 1001, and the second insulating layer 1003 is formed on the first insulating layer 1002. The island-like semiconductor layer 1004 is formed on the second insulating layer 1003 so as to cross the first insulating layer 1002. That is, below the island-like semiconductor layer 1004, there are provided a region where the second insulating layer 1003 is formed, and a region where an insulating film consisting of the first insulating layer 1002 and the second insulating layer 1003 is formed. In the latter region, the volume and heat capacity increase; therefore, the maximum temperature attained by irradiation of laser light is lower than that in the former region. As a result, a crystal nucleus is preferentially generated in the latter region, and crystal growth starts in this region. At this time, it is important to irradiate the semiconductor layer through both surfaces thereof with laser light so as to sufficiently heat the semiconductor layer. Thus, a cycle of a change in temperature by irradiation to the island-like semiconductor layer with pulse laser light is rendered gentle, whereby crystal grains can be made larger.
A method of irradiating a substrate having an island-like semiconductor layer formed on its one surface with laser light through the front and back surfaces of the substrate may also be performed as shown in FIG. 4. Light emitted from a laser oscillator 401 such as excimer laser or YAG laser is split by a cylindrical lens array 402 (or 403). The split laser light is once condensed by a cylindrical lens404 (or 405) and spreads. Thereafter, the laser light is reflected by a mirror 408. A beam splitter 406 is placed in this optical path to divide the optical path into two. The laser light in one of the optical paths is reflected by mirrors 407 and 413. Then, it is formed into linear laser light by a cylindrical lens 414 and irradiated to the front surface of a substrate 418. This laser light is defined as a first laser light. On the front surface of the substrate 418, an underlying film 419 and an island-like semiconductor layer 420 are formed. The laser light in the other optical path is reflected by mirrors 408, 409, and 411. Then, it is formed into linear laser light by a cylindrical lens 412 and irradiated to the back surface of the substrate 418. This laser light is defined as a second laser light. An attenuator 410 is provided in this optical path so as to adjust the intensity of laser light. Even when the substrate is irradiated with laser light through the front and back surfaces in this structure, crystal grains of the semiconductor layer can be made larger in the same manner as described above.
In this specification, laser annealing having the structures as shown in FIGS. 1 and 4 is referred to as dual beam laser annealing, and crystal grains of an island-like semiconductor layer are made larger by adopting this method. By manufacturing a semiconductor device including TFTs having a structure in accordance with the function of each circuit, utilizing the island-like semiconductor layer as an active layer of a TFT, the performance of the semiconductor device is enhanced.
According to the structure of the present invention using dual beam laser annealing, a stripe-like first insulating layer is formed on one surface of a light-transparent substrate, and a second insulating layer is formed on the stripe-like first insulating layer. An island-like semiconductor layer formed on these insulating layers is formed so as to cross the stripe-like first insulating layer. In a preferred embodiment of the present invention, a plurality of stripe-like first insulating layers are formed, an island-like semiconductor layer is formed so as to cross the stripe-like first insulating layers, and a channel forming region of a TFT is formed between a selected stripe-like first insulating layer and its adjacent stripe like first insulating layer.
As described above, according to the structure of the present invention, an island-like semiconductor layer and a strip-like first insulating layer formed below the island-like semiconductor layer are formed on one surface of a light-transparent substrate, and the strip-like first insulating layer is provided so as to cross the island-like semiconductor layer. Paired first insulating layers may be formed, and the pair of strip-like first insulating layers are provided so as to cross the island-like semiconductor layer.
The above-mentioned structure can be preferably applied to a TFT. The above-mentioned region for forming a channel of a TFT formed in the island-like semiconductor layer is formed adjacent to the strip-like first insulating layer, or the channel formation region is formed between a pair of strip-like first insulating layers.
Furthermore, a method of manufacturing a semiconductor device of the present invention includes the steps of: forming a strip-like first insulating layer on one surface of a light-transparent substrate; forming an island-like semiconductor layer over the strip-like first insulating layer so as to cross it; and irradiating the island-like semiconductor layer with laser light through front and back surfaces of the light-transparent substrate to crystallize the island-like semiconductor layer.
Furthermore, another method of manufacturing a semiconductor device of the present invention includes the steps of: forming a pair of strip-like first insulating layers on one surface of a light-transparent substrate; forming an island-like semiconductor layer over the pair of the strip-like first insulating layers so as to cross them; and irradiating the island-like semiconductor layer with laser light through front and back surfaces of the light-transparent substrate to crystallize the island-like semiconductor layer.
Thus, the invention described herein makes possible the advantage of providing a TFT capable of being operated at a high speed by manufacturing a crystalline semiconductor layer in which the position and size of crystal grains are controlled, and using the crystalline semiconductor layer for a TFT channel formation region.